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  • Python如何实现单例模式?其他23种设计模式python如何实现?

    #使用__metaclass__(元类)的高级python用法  
    class Singleton2(type):  
        def __init__(cls, name, bases, dict):  
            super(Singleton2, cls).__init__(name, bases, dict)  
            cls._instance = None  
        def __call__(cls, *args, **kw):  
            if cls._instance is None:  
                cls._instance = super(Singleton2, cls).__call__(*args, **kw)  
            return cls._instance  
     
    class MyClass3(object):  
        __metaclass__ = Singleton2  
     
    one = MyClass3()  
    two = MyClass3()  
     
    two.a = 3  
    print one.a  
    #3  
    print id(one)  
    #31495472  
    print id(two)  
    #31495472  
    print one == two  
    #True  
    print one is two  
    #True
    #使用装饰器(decorator),  
    #这是一种更pythonic,更elegant的方法,  
    #单例类本身根本不知道自己是单例的,因为他本身(自己的代码)并不是单例的  
    def singleton(cls, *args, **kw):  
        instances = {}  
        def _singleton():  
            if cls not in instances:  
                instances[cls] = cls(*args, **kw)  
            return instances[cls]  
        return _singleton  
     
    @singleton  
    class MyClass4(object):  
        a = 1  
        def __init__(self, x=0):  
            self.x = x  
     
    one = MyClass4()  
    two = MyClass4()  
     
    two.a = 3  
    print one.a  
    #3  
    print id(one)  
    #29660784  
    print id(two)  
    #29660784  
    print one == two  
    #True  
    print one is two  
    #True  
    one.x = 1  
    print one.x  
    #1  
    print two.x  
    #1

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  • 原文地址:https://www.cnblogs.com/apollo1616/p/9776737.html
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